Zinc nitrite is known to decompose at around 100° C. with release of nitrogen oxide when slowly heated in air. Also, zinc nitrite is known to be soluble in water and vulnerable to hydrolysis and forms zinc oxynitrite [ZnO.Zn(NO2)2] upon evaporation of its aqueous solution.
Generally, such zinc nitrite is prepared by a method of mixing zinc sulfate with an ethanol solution of sodium nitrite, filtering the resulting precipitate, and evaporating and concentrating the filtrate to obtain crystals (see “Kagaku Daijiten” published on Mar. 15, 1984 by KYORITSU SHUPPAN CO., LTD.).
However, this method, which is a method in which sodium sulfate is separated and removed by utilizing the difference in solubility between zinc nitrite and sodium sulfate, requires the use of ethanol, and the operations of evaporation and concentration. The method has problems in that it inevitably causes an increase in cost, sodium ions are also allowed to remain, and the like.
Therefore, it is currently difficult to obtain high purity zinc nitrite or its aqueous solution on an industrial scale.
Further, as a pretreatment process prior to painting of a metal surface, a series of steps of degreasing, washing with water, film forming treatment, washing with water, and drying is generally included. As an example of the film forming treatment, a treating method for forming a film of zinc phosphate on the surface of steel is commonly adopted. As a film-forming agent used for this purpose, a treating liquid prepared by dissolving zinc in phosphoric acid and diluting the resulting solution with water has been used. This treatment is termed “metal surface treatment”.
Furthermore, in order to promote the film forming reaction of metal, a chemical such as sodium nitrite or sodium chlorate is added to the film-forming agent. These chemicals are called “promoters”. Addition of these promoters shows an effect that the forming treatment can be performed at a lower temperature, and the film forming treatment time is reduced.
However, conventional sodium salts such as sodium nitrite and sodium chlorate have a problem in that long usage of a treating bath increases the concentration of Na ions and as a result the pH of the treating bath is elevated so that components of the formed film precipitate in the treating bath. Also, there is a problem in that, when recovering and regenerating the old treating liquid, accumulation of Na ions in the treating bath destroys the balance of the bath so that removal of Na ions from the recovered treating liquid is necessary. Usually, treating liquid containing Na ions must be disposed of as industrial waste.
Further, the issue of environmental protection has recently attracted much attention even in the field of metal surface treating liquids, and attempts are being made to establish a closed system for treating baths.
On this account, more intensive studies than ever before are being made of a metal surface-treating method with a lower generation of sludge.
The present inventors proposed an aqueous zinc nitrite solution distinguished as a film-forming promoter for a metal surface treatment, which contains substantially no sodium ions and no sulfate ions, and is obtained by reacting between zinc sulfate and calcium nitrite at first and then conducting purification (Japanese Patent Application No. 2000-141893). In addition, it is known that the presence of calcium ions in the film-forming promoter causes the promoter to become sludge as calcium phosphate in the surface-treating solution, for example, at the time of mixing with a zinc phosphate film-forming solution. Usually, the sludge is periodically recovered to be prevented from accumulating in a treating bath. However, the operation of collecting the sludge is also complicated, thereby failing to show an industrial advantage.
The present inventors have been dedicated to making repetitive studies of an aqueous zinc nitrite solution useful as a film-forming promoter for a metal surface treatment, and as a result discovered a nitrite aqueous solution having a reduced amount of sodium ions, in particular, containing substantially no sulfate ions and no calcium ions.
In other words, an object of the present is to provide an aqueous zinc nitrite solution that allows extremely efficient metal surface treatment and has a reduced amount of sodium ions, in particular, containing substantially no sulfate ions and no calcium ions, and a method for preparing the same.